1. Field of the Invention
This invention relates generally to inkjet printers, and more particularly to driver circuitry for matrix inkjet printheads.
2. Related Art
Inkjet printheads for printers are generally classified in two categories: (1) passive and (2) active or "matrix" printheads. Passive printheads comprise a plurality of heater resistors all connected in common to a power supply. The heater resistors are turned on by independent "low" side transistor switches. By contrast, active or matrix type printheads comprise a row and column matrix, where "high" side transistors are provided to couple data to the columns, and a "low" side transistor is provided for each and every heater resistor.
An example of conventional driver circuitry for a matrix type printhead is shown in FIG. 1. In this example, the printhead is a 13.times.16 heater resistor printhead. The plurality of heater resistors are driven in "columns" or groups 20, each comprising 13 heater resistors on the printhead 30. There are 16 groups in this example. The column driver circuit portion 10 comprises a plurality of high side transistors 12 that provide data to the groups 20 of heater resistors R in response to column data signals P1-P16. That is, data signal P1 is assigned to the first group, data signal P2 is assigned to the second group, etc. Heater resistors R in each group 20 are addressed by an associated low side field effect transistor (FET) 22 through another gating FET 24 in response to address signals A1-A13. That is, address signal A1 is coupled to a first row of heater resistors across all of the groups 20, the address signal A2 is coupled to a second row of heater resistors across all of the groups 20, etc. In operation, the printhead 30 is driven by stepping in time through the 13 different rows of heater resistors R and changing the column data signals for each time slice.
The architecture shown in FIG. 1 requires high side transistors for each group 20. These high side transistors require a low Vcesat that cannot vary substantially from a nominal value and furthermore must carry 400 mA. As a result, the column driver circuit portion 10 is relatively expensive. The address driver circuit portion, on the other hand, comprises FETs that are relatively simple (small and low current), inexpensive, and easily integrated on the printhead 30.
It would be beneficial to provide the equivalent column driver signals in a manner similar to that provided by the address or "low" side driver circuit. Preferably, it would be desirable to eliminate the column drivers altogether to reduce driver circuitry costs.